Type Shift+6,7,8,9 (it is "^&*(") during the game to enable cheat mode. If you typed it properly then a message will come saying it is enabled. After that press ALT-C to add 10.000.000 to your credit.
This isn't really a cheat, just a helpful hint. Once you complete the first five campaigns (in either campaign selection) you'll get extra campaigns to do. The more you complete the more choices you get.